The present invention relates to a fuel cell system provided with a fuel cell having a plurality of cells, and more particularly, to control of current when controlling electric power requested by the system.
A polymer electrolyte fuel cells is one example of a fuel cell that generates electricity by utilizing an electrochemical reaction between hydrogen and oxygen. This polymer electrolyte fuel cell is provided with a stack composed of a plurality of laminated cells. The cells that compose the stack are provided with an anode (fuel electrode) and a cathode (air electrode), and a solid polymer electrolyte membrane having ion exchange groups in the form of sulfonic acid groups is interposed between the anode and the cathode.
A fuel gas containing a fuel gas (reformed hydrogen obtained by reforming hydrogen gas or hydrocarbon to make it hydrogen-rich) is supplied to the anode, while a gas containing an oxidant in the form of oxygen (oxidant gas), for example air, is supplied to the cathode. As a result of the fuel gas being supplied to the anode, hydrogen contained in the fuel gas reacts with a catalyst of a catalyst layer composing the anode, and hydrogen ions are generated as a result thereof. The generated hydrogen ions pass through the solid polymer electrolyte membrane causing an electrical reaction with the oxygen. In this configuration, electricity is generated by this electrochemical reaction.
However, in the case of using a fuel cell as a motive power source for an automobile, although the fuel cell is required to be able to accommodate a wide range of usage conditions from low loads to high loads, if the fuel cell is operated under unsuitable conditions, the expected electrochemical reaction is unable to be obtained. For example, if there is a shortage of moisture contained in the polymer electrolyte membrane, the saturated vapor pressure thereof decreases particularly at low temperatures, thereby making it difficult to impart an adequate amount of required moisture to the supply gas.
Therefore, Japanese Patent Application Laid-open No. H7-272736, for example, proposes an invention in which a reaction temperature of a fuel cell is detected, a control table is referred to according to the reaction temperature, a threshold value of the allowable minimum voltage is selected, and when a monitor voltage of the fuel cell is lower than the threshold value, the amount of moisture contained in an electrolyte is determined to be insufficient and a current supplied to a load is interrupted.
On the other hand, in the case of using a fuel cell having a plurality of cells, if even one of the plurality of cells fails to generate electricity, the entire fuel cell may no longer generate electricity. Therefore, Japanese Patent Application Laid-open No. 2003-187842, for example, proposes an invention in which the voltage of each cell is measured, the amount of electric power able to be output by the fuel cell stack is calculated based on the minimum value among the measured cell voltages in the form of a minimum cell voltage, an amount of electric power equal to or less than the amount of electric power able to be output is generated in the fuel cell stack, and when the performance of any one of the cells has decreased, the fuel cell stack is operated in a suitable operating state corresponding thereto.
When the performance of any one of a plurality of cells has decreased, a request current is preferably corrected and controlled by proportional integration (PI), which uses the difference between the minimum cell voltage of a fuel cell stack and a threshold voltage, when operating the fuel cell stack in a suitable operating state corresponding thereto.
However, when correcting a request current in accordance with PI compensation, controllability may decrease if the gain of PI compensation becomes constant depending on the current at the present time.